The invention relates to a method and an apparatus for determining the operational status of a sensor and for engaging the control system of the fuel injection mechanism of the engine. It is known that a particular sensor which determines the oxygen content in the exhaust gas requires sufficient time to be brought to the proper operating temperature and that this period of time is not always the same. The output signals from the sensor which is located in the exhaust gas system affects the function of the fuel injection system, for example by adjusting the duration of the fuel injection control pulses or by engaging the supply of secondary air to the exhaust system.
For monitoring the operational status of an internal combustion engine, it is known for example to use a so-called oxygen sensor or .lambda.-sensor to control the secondary air supply or to directly affect the mixture preparation of the engine. The oxygen sensor is located within the exhaust channel of the engine and, when properly warmed up, provides a signal which contains information regarding the composition of the exhaust gases and hence also of the original fuel-air mixture supplied to the engine. The output voltage of the .lambda.-sensor is similar to a step function and has its maximum slope approximately in the vicinity of the point where the air factor .lambda. is equal to 1. When the oxygen sensor is operating normally, a voltage U.sub.S which is kept constant may be used as a threshold voltage that is compared with the sensor voltage and, depending on which is larger, permits a conclusion as to the original composition of the mixture and its air factor .lambda.. The signal derived from this comparison may be used for further fuel control, as well as for the above-mentioned supply of secondary air and the output signal from the .lambda.-sensor then becomes an actual value which is used in a feedback control loop that includes the engine itself as the controlled variable. During the operation of the control loop, the set point value is altered continuously in accordance with the requirements of the engine.
One difficulty deriving from the operation of the .lambda.-sensor is that it cannot provide information regarding the state of the engine and of the mixture supplied to the engine unless is has reached its proper operating temperature. The .lambda.-sensor, or more precisely its internal resistance, is dependent on temperature. Thus, for example the internal resistance of a cold sensor may be several megohm so that any signal from it would not be usable for a control process.
However, this circumstance may be used to recognize the non-operational state of the sensor by feeding to the sensor a current of suitable and well defined magnitude thereby producing a voltage drop across the sensor which will normally be substantially larger than the threshold voltage and can be appropriately interpreted. For example, this high signal may be used to engage a controller for the secondary air supply by simulating the presence of a rich mixture which causes the secondary air controller to start the supply of secondary air. This air may be admitted to a catalyzer and cause the exhaust gases to be post-combusted. In such a case, the .lambda.-sensor would be disposed behind the catalyzer and would measure the oxygen content of the post-combusted exhaust gases.
On the other hand, the output signal of the .lambda.-sensor may also be used for mixture control, for example when combined with an electronically controlled fuel injection system. In that case, the output signal of a cold .lambda.-sensor might be interpreted to correspond to the richest or the leanest desired fuel mixture. Generally speaking, it is most suitable if the fuel injection system operates at some average value of enrichment when the sensor is non-operational. It is desirable if the average adjustment is also present when the .lambda.-sensor is inoperative for some other reason, for example due to malfunction, or if it is cooled off as a consequence of prolonged idling or overrunning operation and thus is brought back into the temperature region in which the high internal resistance would produce a signal that would cause the fuel injection system to attempt to lean out the fuel mixture to an increasing degree.
When the .lambda.-sensor is reheated, its internal resistance decreases and hence so does the voltage drop due to the calibrated input current. As a consequence, beginning with a certain sensor temperature, this induced voltage drop will be smaller than the threshold value set within the control loop. Once the operational sensor temperature is reached, the threshold within the control loop is exceeded by the sensor signal only whenever the mixture is actually too rich, i.e., when the control process operates normally.
When the .lambda.-sensor is supplied with a constant calibrated current even during normal operation, it causes a voltage drop which may fluctuate substantially during different operational states of the engine due to the resulting temperature changes. This variable voltage drop across the sensor then results in an additional shift of the characteristic curve of the sensor and as a consequence the control process is incapable of high precision. This phenomenon will be discussed in greater detail with respect to FIG. 1 below.